Boyd Mallett's death was probably preventable. He had always been a heavy
smoker and workaholic and had started drinking too much.

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[The Dr Who on telly: Already has a time machine]

His son sank into a despair that would not lift; indeed, he became severely
depressed. Ronald simply could not accept that he would never see his father
again. And he began to wonder if there was a way they could be reunited.

Mallett devoured the pulp sci-fi comics of the time, and began to realise
that time travel was, at least in fiction, a possibility. Then he read what is
one of the finest science-fiction stories ever written, HG Wells's The Time
Machine.

In the novel, the time-travelling hero explains: "Scientific people know very
well that time is only a kind of space. We can move forward and backward in
time, just as we can move forward and backward in space."

Mallett was dumbfounded. If he could build a time machine, he could go back
and change history and prevent his father's death.

From that day, Mallet became obsessed with time travel, despite having no
clear idea of how it could be accomplished.

Wells's book was, of course, entirely fictional, and yet, just a few years
after it was written, a German-Jewish physicist called Albert Einstein blew the
science community apart. Einstein showed that time and space were indeed
different aspects of the same thing - a concept called spacetime - which is at
the heart of how physicists understand the way the universe is.

Mallett became obsessed with the German scientist - who had died in 1955, the
same year as his father. Most importantly, Mallett realised - as Einstein had
himself - that the new way of thinking about gravity, space and time contained
in the physicist's Special and General theories of relativity meant that a time
machine was at least possible in theory.

Einstein's equations showed that by twisting spacetime around, it is possible
in theory to make a connection from future to past. Step into this timeloop, and
you could emerge years later or earlier.

This idea would form the basis of Mallett's putative time machine. But, back
in 1950s New York, he was a long way from his goal. Growing up poor and black,
one of four children raised by a widowed mother who made ends meet by
window-cleaning, is not an ideal recipe for academic success.

Undaunted, he studied hard at school and achieved good grades, particularly
in the sciences. However, a university education was out of the question - there
was simply no way his family could afford to pay for it.

So Ron Mallett joined the U.S. Air Force, in the hope of being granted a
military scholarship so that he could later study physics. His test grades were
so good that he was fast-tracked into the USAF's electronics school.

Despite his success, the past still intruded in the most horrible ways.
Mallett's first tour of duty was in Biloxi, in the Deep South. There, for the
first time in his life, he encountered the soul-destroying racism that had
driven his grandparents north 40 years before.

"The first thing I noticed," he writes, "were the signs, the likes of which I
had never seen before. 'Whites only'. 'No Colored'."

There was talk of beatings and worse for black servicemen who strayed off
base. Mallett made a vow to remain on base for the entire duration of his
training, which included courses in electronics and computing. He also spent
hours in the well-equipped library, devouring everything he could both by and
about Einstein.

His studies paid off. After he was discharged, he won a place at Pennsylvania
State University, and began a degree in physics.

Eventually, in 1973, he won his doctorate, only the 79th black American ever
to do so in this subject. Part of his thesis was an investigation into the
theoretical possibility of using gravity to reverse the passage of time. In
1975, he was awarded a job as a professor of physics at Connecticut University -
where he has worked ever since.

CORRECTION: Mallett is a professor of physics
at the University of Connecticut.

He remains the only black physics professor in America.

CORRECTION: Mallett is one of a small number
of African-American physics faculty members in the United States of America".

Despite the respectability of his CV, he still felt he couldn't discuss his
ideas openly. "I feared professional suicide," he says now.

But, as his work continued, the story got out. Mallett's time machine went
public in 2001, when New Scientist magazine ran an article about his design, and
TV appearances followed.

"Mallett isn't mad," the New Scientist article said. "None of the known laws
of physics forbids time-travel.

"In theory, shunting matter back and forth through time shouldn't be that
difficult."

So, how do you build a machine which will take you back into the past - or
forward to the future?

In fact, there have been several plans for a time machine devised by
physicists since Einstein's mind-blowing discovery that reverse timetravel
should be possible.

In 1974, Frank Tipler, a physicist at Tulane University in New Orleans,
calculated that by constructing a huge cylinder in space and setting it
spinning, it would be possible to drag spacetime into loops, creating lots of
backwards time portals into which you could leap and then emerge in the past.

But he calculated that the cylinder would have to weigh about as much as the
sun, and be compressed into a tube 60 miles long and 40 miles across.

Alternatively, as physicist Kip Thorn proposed in the 1980s, you simply need
to create a 'wormhole' - a tear in the fabric of spacetime, using perhaps a tame
black hole or dozens of nuclear bombs.

These ideas, while scientifically correct, were hardly practical. Squashing
the sun into something the size of Dorset is likely to be beyond our ken for
some time, and harnessing the power of a black hole sounds even harder.

Mallett's solution is much simpler. He thinks he can reverse time by using
just a circulating beam of light. Light is energy, and energy can cause
spacetime to warp and bend, just like gigantic spinning cylinders, he explains.

In 2000, he published a paper showing how a circulating beam of laser light
could create a vortex in spacetime. It was, he says, his eureka moment.

The details are complex, to say the least. But, in essence, Mallett believes
it is possible to use a series of four circulating laser light beams swirling
spacetime around like "a spoon stirring milk into coffee".

If you were to walk into this 'timetunnel' - which would resemble a large
vortex of light a few feet across - you could emerge at some point in the past.
He thinks he can build a prototype machine in the lab, using today's technology,
with funds of just $250,000 (£120,000).

However, Prof Mallett is fussy about who gives him the money. "We want
non-military sources. I don't want to get to a certain point and get 'top
secret' slapped over the project and have it taken away from us."

There are several important things to realise about Mallett's time machine.
For a start, it would only be possible to travel back in time to a point after
the machine was first switched on.

If you turned on the machine, on January 1 say, and left it running for three
months, you could enter the machine in March and only travel back as far as
January 1.

So no trips back to the Middle Ages or to Ancient Rome.

This would be staggering enough. Just think: a time-traveller could go back
and meet himself. Or he could send back information into the past - including
the results of horse races, stock market movements.

But consider, too, all the weird paradoxes that the time machine would
create. You could come face-to-face with your past self, causing untold
confusion. What, for example, would happen if you killed your past self? Would
both versions of 'you' die at the same time?

Mallet believes these paradoxes would not in themselves prevent the
construction of such a machine. But there are plenty of sceptics.

Some physicists think that the laser upon which his machine depends would
need to be impossibly large or powerful. Others point to Stephen Hawking's
'chronology protection conjecture', which says that quantum effects may conspire
to prevent the possibility of a time machine.

But, while some physicists have questioned Mallett's approach, no one has yet
proved with absolute certainty that the machine would not work.

Mallett is now 62 years old. He still believes he will live to see the
creation of the first time machine.

Sadly, the way it works means that he will never be able to fulfil his
original wish - to warn his father about his deteriorating health. "My solace is
that if this works, future generations will be able to use this technology to
prevent the tragedy that I went through," he says.

If he is right, the little boy from the Bronx who lost his beloved father all
those years ago will end up being the most famous inventor in history.

The Time Traveller: One Man's Mission To Make Time Travel A Reality, by
Ronald L. Mallett with Bruce Henderson, is published in August by Doubleday at
£14.99. To order a copy (p&p free), call 0870 161 0870.